Conventionally, in a library system, as illustrated in FIG. 6, a host directly controls a robot path to control mounting or unmounting of a storage medium (for example, a tape medium), and reads or writes data via a physical drive in a library device (i.e., tape library).
Virtual library systems have been widely used that include a virtual tape device arranged between a host and a library device. The virtual tape device provides a logical drive and a logical volume to the host to read data from the host or write data more quickly (see Japanese Laid-open Patent Publication No. 11-102262). Specifically, in a virtual library system, data stored in a logical volume is retained in a cache disk, and then a migration process is performed for storing, in a physical volume of a library device, the data stored in the logical volume not in synchronization with a host access. The logical volume saved in the physical volume in the library device is retained as long as the capacity of the cache disk permits.
There is a demand that, to prevent a data loss due to a disaster, such a virtual library system has a redundant configuration including a main center and a sub-center and data is synchronized between the main center and sub-center (see Japanese Laid-open Patent Publication No. 2005-267216 and Japanese Laid-open Patent Publication No. 2006-48103).
Among methods for synchronizing data between a main center and a sub-center, for example, a technology is known in which a portable medium is transferred to the sub-center to synchronize data between the main center and the sub-center. Specifically, as illustrated in FIG. 7, data is synchronized between the main center and the sub-center in a way that an export process for saving data stored in a logical volume in a physical volume in a library device is performed in the main center, the storage medium of the library device is then transferred to the external sub-center, and an import process is performed for importing the data stored in the physical volume in the transferred storage medium to a logical volume of the sub-center.
As a method of synchronizing data between a main center and a sub-center, a technology is also known for synchronizing data between a main center and a sub-center by performing a data triple saving process. Specifically, as illustrated in FIG. 8, data triple saving is performed in a way that a library device is arranged in the sub-center and a migration process using a logical volume in a virtual tape device of the main center is performed as required on the physical volumes in the library devices of the main center and the sub-center. If a disaster occurs in the main center, the sub-center performs a disaster recovery to synchronize data between the main center and the sub-center.
In the technology for synchronizing data between the main center and the sub-center by transferring the portable medium to the sub-center, however, because recovery is performed by performing the import process on the data of the storage medium that has undergone the export process and has been transferred, the export process cannot be performed after the transfer. This leads to a problem in that data retained from the time when the process is performed to the time when a disaster occurs in the main center is lost. This also leads to a problem in that costs are incurred and time is required for transferring the portable medium on which the export process has been performed.
In the technology for synchronizing data between the main center and the sub-center by performing the data triple saving process, the migration process for reflecting an update of the logical volume in the virtual tape device in the physical volume in the library device is performed as required. This leads to a problem that, if a disaster occurs in the main center during the migration process, the update of the logical volume in the virtual tape device is not completely reflected in the physical volumes of the library devices in both the main center and the sub-center, and thus data is lost.